Keeping Better Time With Atomic Clocks: A Live-Blog Event With Nobel Laureate David Wineland

If you want to mark the passage of time, you need something to occur with regularity, repeatedly, that you can use as your measure of keeping time. Throughout history, we've really had two options for this:

to use an artificial device made by humans, such as a mechanical, chemical, or atomic device,

or to use a natural astronomical device, such as a planet, moon, star or pulsar.

Image credit: ESO/L. Calçada.

For a long time, the best clocks known to humanity were the pulsars: neutron stars that rapidly rotate -- up to 766 times per second -- with remarkable precision, to the point where you could look away for years and then look back, and know whether ten-billion pulses had taken place, or whether it was ten-billion-and-one. We've been able to use pulsar timing over long baselines, years or even decades, to perform timekeeping to fourteen or fifteen significant digits.

But despite the incredible accuracy of these natural clocks, it was the artificial atomic clocks that came to prominence in the 1980s and 1990s, passing the pulsars and becoming the best timekeeping tool in history.

Image credit: Perimeter Institute / Geoffrey Wheeler.

To help you understand why atomic clocks rule, let me give you a bit of an introduction. For centuries, we had defined quantities like mass through a block of metal that consisted of one kilogram, time through astronomical means like the orbit of the Earth around the Sun, and the speed of light by measuring it as accurately as possible in terms of those other two quantities. But we realized over the past century that these methods are woefully inadequate if we want to go to incredibly high precision. So what were we to do? We went down to the building blocks of everything we are: the atom itself.

Image credit: Wikimedia Commons user PoorLeno.

When an electron transitions from an excited state to a lower-energy state, it emits a photon (a particle of light) of a very particular wavelength. We define the speed of light to be exactly 299,792,458 meters/second, and then:

a meter can be defined by the length a specific number of waves of a certain transition take up in vacuum,

and a second can be defined by the time it takes a photon to travel that one meter.

In order to make the most accurate atomic clock possible, you need those atoms to be as cold as possible (to eliminate thermal noise), as motionless as possible (to eliminate kinetic noise), as recoilless as possible, and -- if you want to improve on the limits of a single atom -- in a system that consists of many such atoms to improve your statistics.

Image credit: Perimeter Institute / NIST.

Later today, November 4th, 2015, at 7 PM Eastern / 4 PM Pacific, the 2012 Nobel Laureate David Wineland, pioneer in the field of atomic clocks and one of the premier researchers in the field, will give Perimeter Institute's public lecture on the topic of Keeping Better Time, including how one builds an atomic clock, how the best artificial clocks have progressed to be more and more accurate over longer timespans, and what the future of timekeeping holds. (Trailer here.)

You can watch the lecture live, right here, in the window below.

I highly recommend keeping this window open in a separate tab from the lecture, as well, because this page will be updated every few minutes during the live-blog with commentary and updates from me, Ethan Siegel, a theoretical astrophysicist who's a huge fan of pulsar timing; it's one of my active areas of research. Will astrophysical sources ever make a comeback? Find out at 7 ET / 4 PT tonight!

Image credit: Wikimedia commons users LucasVB and Kalki.

3:48 PM (all times Pacific): Let's get started early! One of the most exciting things about using light to keep time is that you actually don't need a perfect vacuum in order to make it happen! Light has this remarkable property that once it leaves the source, its frequency remains unchanged, even as it travels through various media. If you can measure the number of "waves" that occur, you can measure time as precisely as you like, regardless of what that light travels through.

3:52 PM: Atomic clocks, for those of you picturing a huge, monstrous and unwieldy application, are actually well within the reach of us all. In 2004, they became developed to be "chip size," which is a technology that became commercially available in 2011. The device power draw on a chip-sized atomic clock is 125 mW, or just one eighth of a Watt. They can be found in a huge variety of devices, including almost all modern GPS's.

3:57 PM: Are you wondering when atomic clocks passed pulsars for most accurate timing?

Image credit: NASA/GSFC (main); NIST (inset).

It was just before the development of laser cooling. Pulsars can be accurate to one part in 10^14 or 10^15, but atomic clocks have long since passed that, and are now at the one part in ~10^17 level, far surpassing what pulsars can do. But wait long enough, and pulsars will continue to improve! So long as they don't glitch, if you wait 100 times as long, you're 100 times more accurate.

4:00 PM: This is the first live-blog I've done for Perimeter, by the way, since we moved to Forbes. So far, all systems go!

4:03 PM: What are the biggest advances in atomic clocks? Ion traps, cooling techniques, innovations in single-atom techniques, as well as creating entangled multiple-atom systems. Can't wait to hear what we'll hear from David Wineland... here we go!

Image credit: screenshot from Perimeter institute live stream.

4:05 PM: That's a fun challenge: Wineland promises that if he does his job correctly, you can build your own atomic clock with one minute of information from his lecture!

4:07 PM: This is an important part of history: you need time for navigation purposes. If you can measure the position of stars or the Sun, you can know your latitude pretty straightforwardly. But for longitude? You need time: you need precision timekeeping to know where you are. The excellent science book, Longitude by Dava Sobel, gives a remarkable historical treatment of this subject.

Image credit: Perimeter Institute's live stream.

4:10 PM: Wineland tells us that in the 1700s, the problem of measuring time to know your longitude was such a large problem that many rewards were offered. Here's a fun story: Christiaan Huygens, the pioneer of wave theory, discoverer of Saturn's moon Titan and all-around polymath, was also the finest pendulum clockmaker of his day. He built a great one in Holland that kept time so perfectly it was decided it'd be shipped to the new world to keep time there. Yet when it arrived, it was off: by about 30 seconds a day, but that was significant! So they shipped it back to Holland for repairs (remember, this was the 1600s), where it went to run perfectly, and needed no repairs at all! The reason, unknown to all, is that the Earth bulges at the equator, meaning that "g", or the Earth's gravitational acceleration, was different in the Caribbean than it was in Holland. Who knew?!

4:13 PM: Crazy to learn that, for human-made clocks, pendulum clocks were literally the best we could do until atomic clocks came along; there was nothing better in all that time!

Image credit: Perimeter Institute's live stream.

4:15 PM: Here's the key to an atomic clock:

Start with an atom in the ground state.

Shine photons on it that have a probability of exciting that atom at the atom's natural oscillation frequency.

This maximizes the absorption probability.

When the excitation happens, and then the electron drops back down, it emits a photon.

When the emission happens, you get a "count," and that count enables you to keep time.

Not so bad, right?

4:18 PM: The most important thing to count, though, is the oscillations. So why are atomic clocks better than pendulum clock? Because pendula only depend on two things: "g", or the local acceleration of Earth's gravity, and "L", or the length of the pendulum. But if the temperature changes even a little, including the temperature of the bob itself, it can cause a lengthening or shortening. And even if that changes by a part in a million or a part in a billion per degree, that limits your accuracy to about one part in 10^8. But atoms can do much, much better.

4:21 PM: One of the great things about atomic clocks is that relativistic effects, including very, very slight motions, need to be taken into account. If you -- the measurer -- aren't in exactly the same reference frame as the clock, you'll need to take those relativistic corrections into account. This is also why you want to minimize thermal noise (which causes motion) and recoils (which is a motion). Any imperfection in frequency you induce will potentially mess up your timing, but these are all things we can address and improve upon experimentally!

Image credit: Perimeter Institute's live stream.

4:24 PM: Trivia time: what atomic transition defines the second? The Cesium-133 atom, which has a hyperfine transition that emits a photon of a very particular wavelength, and 9,192,631,770 cycles of that wave defines the second!

Image credit: Perimeter Institute's live stream.

4:28 PM: If you go to high frequencies -- higher energy transitions -- you can get more and more precise measurements of time! Why's this? Because narrower frequencies have less uncertainty (or "width") to them, and therefore a single measurement tells you much, much more information. Rather than Cesium, going to Mercury atoms allows about a factor of 10^5 improvement in how precise an atomic clock can be. They're called "optical" clocks because it's the photons emitted and absorbed, rather than the atoms themselves, that we use to determine time.

4:31 PM: Fun fact: Mercury's transition for a good atomic clock occurs when it's ionized. Wineland referenced, earlier, that there is some very good work going on with neutral atoms, but that ions are the presently best clocks. Why? It's all a question of where you get the finest, highest-energy, narrowest-frequency transition, which happens to occur in single-atom, ionized systems. (As discovered so far!)

Image credit: Perimeter Institute's live stream.

4:33 PM: Here's a simple one: why do we use single atoms? (Or single ions?) These give the smallest frequency shifts... and we can still measure the absorption/emission from these atoms/ions!

4:35 PM: WHAT?! Did you just catch that little throwaway comment Wineland made? There's a group using Barium as their atomic clock "single atom," and it has a transition that's blue. As in, visibly blue. He says it looks like a faint star. So just to recap, we have an atomic clock that you can see with your eye, a single-atom clock, and it looks like a star. Eat your heart out, Twitter.

Image credit: Perimeter Institute's Live Stream, David Wineland.

4:38 PM: As anticipated, cooling is key. Note that "ion loss" at relatively high (room) temperatures is frequent, on the order of seconds. But a very, very cold trap -- at 4 K (liquid helium) temperatures -- can hold single ions for arbitrarily long amounts of time, like 6 months. (Which could've been much longer.) If you have a longer baseline of observation/measurement, you can beat down your uncertainties as the inverse of the square root of observing time. So if you observe 1,000,000 times as long, your clock is 1,000 times as accurate. Good job, atoms!

4:42 PM: Every seven seconds in his data of atomic clocks -- one-seventh of a Hz -- you see a periodic shifting in your frequencies if you have two reflective surfaces (mirrors) that your light zips between. Why? Because this laboratory is located on the California coast... and that's the frequency of waves crashing on the shore. That's a sensitive experiment!

Image credit: Perimeter Institute's live stream.

4:46 PM: The biggest things you need are a way to make these pulses, to hold the atoms in place, and to count these transitions at very high frequencies. The development of laser technology, the identification of high frequency transitions and the cooling to reduce the relativistic time shifts has enabled the tremendous -- factor of a million or so -- advances of the past 30 years or so. These advances are how Mercury finally defeated Cesium in the 2000s.

4:49 PM: Another throwaway, which is super important to anyone who wants to win a Nobel Prize (easy pickings) so long as you have unlimited power and money: instead of using atomic transitions (i.e., electron transitions in atoms and ions), use nuclear transitions, which doesn't have visible or ultraviolet transitions, but has gamma-ray transitions, at frequencies that are many orders of magnitude higher than the ones atoms/ions use. If you can build a gamma-ray laser of the right frequency, this has the potential for improving timing by factors of many thousands. Thousands!

4:52 PM: In the meantime, not only have Mercury ions passed Cesium atoms, but Aluminum (or aluminium) ions have passed mercury, for reasons that Wineland went too fast for me to parse! (Sorry!)

Image credit: screenshot from Perimeter Institute's live stream.

4:57 PM: Does gravitational redshift exist? Yes! Here's a fun thing: if you raise your experiment by 33 centimeters (0.33 m), it causes a frequency shift that's super, super tiny, but measurable! This was how the Pound-Rebka experiment (which was in the late 1950s and greatly less sophisticated) first measured the gravitational redshift phenomenon on Earth!

4:59 PM: This has an interesting consequence: if you want to compare the results from two clocks, you need them to be at the same location! Limiting, isn't it? Thanks a lot, Einstein!

Image credit: Screenshot from Perimeter Institute's live stream.

5:01 PM: And this is very important: there is the potential to probe fundamental physics -- or constrain deviations from what's expected -- by using atomic clocks. In particular, the relative strengths of the fundamental forces may change over time, and more precise clocks will help probe that. Similarly, Einstein's predictions for the magnitude of, say, gravitational redshift, might differ from reality if we get down to enough significant figures. That's more than enough motivation to keep going!

5:02 PM: And the talk is over, but here's a David Wineland quote from the end of his talk to get you thinking:

"Through the centuries, whenever there has been a better clock, there's been a use for it."

5:04 PM: Here's a good question from the Q&A: does the uncertainty principle pose difficulties when you try to hold an atom still? Well, yes, the uncertainty principle restricts us from constraining the single atom/ion to a point, but at a finite size, we're okay. Why? Because we're constricting it's momentum, because we're interacting with it, and because we're making measurements that keep the position wavefunction from spreading out arbitrarily.

5:07 PM: A little disappointed, as a recap, that he never talked about why you would want to use multiple atoms until right now. You would get more signal this way, with multiple atoms, but the atoms could perturb one another, and that risk -- at present -- and the errors it causes, is larger than the benefit you get with multiple atoms.

5:09 PM: Rather than detect gravitational waves directly, multiply positioned atomic clocks -- widely separated -- could allow you to detect spacetime distortions due to gravitational waves by measuring the frequency/timing deviations from the expected behavior. The technology isn't there yet, but there is a potential way to use atomic clocks for gravitational wave astronomy!

5:11 PM: And that's the end! Thanks for joining us, and of course you can watch the live stream (recorded version) in perpetuity here, and follow along with the comments/live blog for extra information at (almost) every turn.

Astrophysicist and author Ethan Siegel is the founder and primary writer of Starts With A Bang! His books, Treknology and Beyond The Galaxy, are available wherever books are sold.